May 3, 2000
12:00 AM (EDT)

News Release Number: STScI-2000-18

Lost and Found: Hubble Finds Much of the Universe's Missing Hydrogen

May 3, 2000: For the past decade astronomers have looked for vast quantities of hydrogen that were cooked up in the Big Bang but somehow managed to disappear in the empty blackness of space. Now, NASA's Hubble Space Telescope has uncovered this long-sought missing hydrogen. This gas accounts for nearly half of the "normal" matter in the universe -- the rest is locked up in galaxies. The confirmation of this missing hydrogen will shed new light on the large-scale structure of the universe. The detection also confirms fundamental models of how so much hydrogen was manufactured in the first few minutes of the universe's birth in the Big Bang.

Q & A: Understanding the Discovery

1.
How did Hubble detect this elusive hydrogen?

This hydrogen is so hot it escapes detection by normal observational techniques. So astronomers used Hubble's Space Telescope Imaging Spectrograph to hunt for oxygen, an element that mixed with and was heated by the hydrogen.

Astronomers detected this highly energized oxygen by using the light of a distant quasar to probe the invisible space between the galaxies, which is like shining a flashlight beam through a fog. The imaging spectrograph found the spectral "fingerprints" of intervening oxygen imprinted on the quasar's light. Slicing across billions of light-years of space, the quasar's brilliant beam penetrated at least four separate filaments of the invisible hydrogen laced with the telltale oxygen. The presence of oxygen between the galaxies implies there are huge quantities of hydrogen in the universe.

2.
How did the hydrogen heat up?

Supercomputer models of the expanding, evolving universe have predicted an intricate web of gas filaments where hydrogen is concentrated along vast chain-like structures. Clusters of galaxies form where the filaments intersect. The models predict that vast hydrogen clouds flowing along the chains should collide and heat up. This would squelch the formation of more galaxies in the hottest regions, so star birth was more abundant in the early universe when the hydrogen was cool enough to coalesce. Observations with ground-based telescopes previously detected vast clouds of relatively cool hydrogen between galaxies in the early universe.

3.
How was the oxygen created?

Exploding stars in galaxies probably spewed the oxygen into space where it mixed with the hydrogen.